1. Field of the Invention
Curable coating compositions based on polyamide powders.
2. Description of Related Art
Curable coating compositions based on a hydroxyl-functional binder and a polyisocyanate or a polyester-melamine resin system which comprise a suspended finely divided polyamide are known (EP B 0 047 58). These coating compositions cure to give a weather-stable and light-stable coating possessing good mechanical deformability, good abrasion resistance, and good chemical resistance.
At the present time fine polyamide powders principally based on polyamide 11 and polyamide 12 are used in polyurethane- and polyester-based coating materials as texturing agents and property enhancers. Other binder systems as well are conceivable.
These systems find application primarily in the coil coating industry, but also in the segment of wood coatings and, increasingly, in general industrial sectors. Examples include floor coverings or abrasion-resistant wood coatings for kitchen furniture (worktops).
In the present-day applications use is made of the very good abrasion resistance of the polyamide powders, particularly as texturing agents, in order to extend the life of the coating systems, in the area of sun protection systems (roller shutter bars), for example. In the coil coating process the coating systems are applied to sheets of aluminium and of steel which are subsequently cut and shaped to give the desired components.
The coating compositions developed to date are, for example, reactive systems which are reacted, and cure, in baking ovens, an example being the deblocking of isocyanates at temperatures above 160° C. and subsequent reaction with terminal hydroxyl groups of a polyester.
The basis for improving the polyurethane and/or polyester coating system with polyamide 11 powders and/or polyamide 12 powders as texturing agents is the reaction of the free reactive terminal groups which are demonstrably present in the polyamide, primarily terminal carboxyl groups and terminal amino groups, with the reactive components generated in the oven. This is accompanied by chemical incorporation of the polyamide into the coating film which forms within the oven.
A further suitable component for influencing the surface qualities and abrasion resistance of coating films are waxes, particularly micronized PE waxes. From a performance standpoint, however, high concentrations of waxes have adverse effects in the coating film. The coatings frequently exhibit a high sensitivity to fingerprints. Exudation phenomena may occur at the surface, which can reduce the brightness of the coating or even lead to a milky clouding of the coating. It has also been observed that high proportions of PE wax are dispersed incompletely in the coating material. As a result of the inhomogeneous distribution, the finished coating film has a spotted texture.
The object was therefore to provide a coating composition which does not have the described disadvantages even in the case of high wax fractions. This object has been achieved as described below.